In order to impress on christians the obligation of brotherly-love, John again reminds them, that through God's love to them their own love was first enkindled; and then goes on to show, that in love to God is necessarily involved love to the brethren. "We love him because he first loved us. If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?"

The appeal thus founded on that conscious christian fellowship, to which both he and his readers have been admitted, presupposes this love to God as something possessed in common, originating as it does in their common experience of God's redeeming love. But so certainly as this love exists among them, will it reveal itself as such in its effects. It is easy to say, I love God; the point is, that this love should manifest itself by its unmistakable signs in the life. The witness to this presence of God's love in men, is Brotherly-love. He who says he loves God, and yet hates his brother, is called a liar, since his professions are proved by his acts to be lies; for in John's view, hatred of one's brother and love to God mutually exclude each other. We must here remember, that with John there is nothing intermediate between love and hate; it is either love to the brother, or hatred of the brother. With him therefore, "to hate the brother," and "not to love," are one and the same; since where love is wanting, the selfish disposition already contains the germ of hate.

But is it not strange that John should ask: How can he who loves not the visible brother, love the invisible God? For he always regards love to God as the primary, and love to the brother as the derived affection; self-sacrificing brotherly-love as originating in love to God, that alone being able to overcome the selfish principle in man. But if from the cause, we may deduce its necessary and spontaneous effect; so on the other hand, from the effect we may reason back to the cause, and regard the effect as evidence of the cause. Love to God is in itself an invisible act, seen only by him who looks upon the heart; but the effects of this love, as they appear outwardly, are seen by man. Whether there is true love to God must be determined, therefore, by the presence or absence of Brotherly-love. Hence John's conclusion: How can I believe that he truly loves God, in whom I see not the visible evidence of this love? The visible here bears witness of the want of the invisible. And moreover, man as a creature of sense, is more readily affected by the visible than by the invisible. If we conceive of love as a capacity inherent in the God-related nature of man, and pointing back to its primal Source in God who is Love; yet, for this capacity to raise itself to the Invisible One, more is required than to awaken it into action through the impression made by his visible image in man. How is the invisible object of love to exert an influence upon him, whom the visible leaves unaffected?